Preparation And Functional Characteristics Of Whey Isolate Protein-Dextran Conjugates

Whey protein isolate(WPI)is rich in various essential amino acids and is easily digested and absorbed by the human body.Consequently,WPI is widely used in the food industry.However,Whey protein isolate is susceptible to external factors during food processing,such as temperature,p H,and ionic strength,leading to protein aggregation or precipitation,which cause the loss of its functional characteristics and limit its application areas.The Maillard reaction without addition of toxic chemical reagents is a green and safe method of protein modification.The conventional dry heat method of Maillard reaction has uneven contact and the wet heat method of Maillard reaction undergoes high temperature and is prone to cause protein denaturation.The two methods have problems with high energy consumption and high wastewater yields during the preparation process.In this paper,the advantages of dry heat and wet heat methods were combined.The model of the Maillard reaction between whey isolated protein and dextran in alcohol-water solution was established using ethanol solution instead of partial aqueous solution based on wet heat method.The functional properties of whey isolated protein-dextran conjugates were analyzed.Fish oil microcapsules were prepared by whey isolated protein-dextran conjugates as wall material.The physicochemical properties and stability of fish oil microcapsules were studied.The main research findings are as follows:(1)The grafting degree was selected as the index.The response surface design test of four factors,such as mass ratio of whey isolated protein to dextran,reaction time,reaction temperature,and ethanol volume concentration with three-level was carried out to optimize the alcoholwater pretreatment process based on single factor experiments.The results showed that the influence of each factor on the grafting degree was in the following order: ethanol volume concentration > mass ratio of whey isolated protein to dextran > reaction time > reaction temperature.The optimal process conditions were: mass ratio of whey isolated protein to dextran 1:3,reaction temperature 70°C,reaction time 23 h and ethanol volume concentration 92%.The grafting degree was 24.22±0.35% under the optimal conditions.(2)The structural and functional properties of glycosylated proteins were studied for different reaction levels.The results showed that the intermediates and browning of the conjugates gradually accumulated with increasing reaction time.The formation of whey isolate protein-dextran conjugates was confirmed by both sodium dodecyl sulphatepolyacrylamide gel electrophoresis and spectroscopy.The surface hydrophobicity results suggested that the covalent grafting of dextran had a shielding effect on the hydrophobic groups of whey isolate protein and its surface hydrophobicity was significantly reduced with the increase in grafting degree.Scanning electron microscopy exhibited that the structure of whey isolate protein changed from a typical smooth spherical surface to a rougher blocky surface after the covalent grafting of dextran.Turbidity and solubility indicated that whey isolate protein-dextran conjugates had good p H stability.Compared with unmodified whey isolate protein,the emulsification and emulsification stability of whey isolate protein-dextran conjugates increased by 36.86% and 26.95%,respectively,their reducing power increased by 1.13 times,DPPH and ABTS radical scavenging rates increased to 42.98% and 54.79%.(3)Fish oil microcapsules were prepared by whey isolate proteindextran conjugate as a wall material.The physicochemical properties and stability of fish oil microcapsules were investigated.The results showed that the powder of fish oil microcapsules was homogeneous,no agglomerates and fishy odor.The fish oil microcapsule powder had a moisture content of 3.50±0.44%,a bulk density of 0.23±0.03 g/cm3,a repose angle of 33.70±0.59°,a solubility of 69.08±4.04%,an encapsulation rate of 76.04±2.03 %.FTIR of fish oil microcapsules exhibited that characteristic absorption peaks of fish oil and its intensity reduced,which indicated that fish oil was successfully encapsulated in microcapsules.A smooth and dense surface structure of fish oil microcapsules was revealed by scanning electron microscopy.The glass transition temperature of fish oil microcapsules was determined by differential scanning calorimetry and was 109.27°C.The peroxidation value(POV)of the encapsulated fish oil was 11.45±0.59 meq/kg after one week of accelerated storage at 60°C,which indicated that fish oil microcapsules had good storage stability.The simulated digestion experiment in vitro suggested that the slow release of fish oil encapsulated by whey isolate protein-dextran conjugate could be achieved.